1. Field of the Invention
The invention relates to liquid crystal display device and somewhat more particularly to a process of producing homeotropic orientation layers for use in such devices and the resultant liquid crystal devices.
2. Prior Art
Molecules of liquid crystal layers must be aligned at essentially right angles with an associated carrier or substrate plate in an unactivated or rest state ("homeotropically") for a number of practical applications, such as, for example, in a liquid crystal display device having dynamic scattering or one based on specific field effects (i.e., "DAP-effect", "phase-change effect", etc.). Heretofore numerous different techniques have been developed to provide homeotropic orientation in liquid crystal layers, however, while at least certain of such techniques provide serviceable and generally reproduceable results, they are nevertheless complicated and are only marginally useful for large-scale production. A summary of various, currently utilized orientation techniques is set forth in an article by L. A. Goodman, "Liquid-Crystals-Packaging and Surface Treatments", RCA Review, Vol. 35, Sept. 1974, pages 447-461. In spite of intensive efforts by workers in this art, no effective satisfactory molecular alignment or orientation techniques for liquid crystals has yet been discovered and that may be associated with the fact that the actual orientation mechanisms are not yet fully and/or completely understood so that, as before, one is left to more or less plausible suppositions.
If a liquid crystal layer is electrically activated, such a layer must be protected from any D.C. voltage, which as is known, decomposes liquid crystals. Accordingly, the electrodes of a liquid crystal display device are generally coated with an insulating layer since in virtually all liquid crystal activation techniques, specific D.C. voltage components are unavoidable. An insulating layer or coating of this type must satisfy a series of requirements: it must posses a very high electrical resistance and must be capable of forming a substantially pore-free film which adheres to various types of materials, such as the materials of a substrate or an electrode; it must be mechanically, thermally and chemically stable and must transmit as much light as possible; and, not the least, it must be capable of being economically produced. A layer which simultaneously exhibits a homeotropically orientating function and an electrical insulating function would be substantially ideal.
Insulating layers composed of SiO.sub.2 have, in the past, proven to be capable of meeting some of the aforesaid requirements. However, SiO.sub.2 orientates liquid crystal materials, if at all, only in a plate-parallel fashion ("homogeneously"). If a plate-perpendicular or homeotropic molecular alignment is required and if it is not desired to dispense with the SiO.sub.2, which, in other respects, is so-favorable, the SiO.sub.2 surface would have to be silanized (i.e. provided with a layer of silanol) and/or the liquid crystal layer would have to be admixed with some other orientation means. This type of procedure is extremely complicated and is certainly not beneficial to the quality of the resultant liquid crystals. A further disadvantage of this type of procedure is that the disruptive reflections from a double layer consisting of SiO.sub.2 and silanol groups can only be avoided by careful matching of layer thicknesses, which is difficult to achieve.
German Auslegesschrift No. 2,313,730 (which generally corresponds to U.K. Patent Specification No. 1,428,700) states that a SiO.sub.2 or TiO.sub.2 layer positioned between an electrode and a carrier plate of a liquid crystal display device provides homeotropic orientaion via specific surface treatment (i.e. etching away portions of the overlying conductive layer). Apart from the fact that such specific surface treatment process represents an additional operational step, the so-produced orientation effect does not appear to be readily reproduceable.